Updated openstack/openstack
Project: openstack/ceilometer  12e3266263c6cc8ec9f88319f72efd5e87c4a2b9

refresh ceilometer architecture documentation

there are stale descriptions in the architecture documentation
compared to what currently exists. this patch:
- updates/removes incorrect details
- updates architecture diagram
- makes note of horizontal scaling design

Change-Id: I5debdcb2c34d0c2e1e637d3d75db0dfbc44b9124